1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transmissive and reflective type liquid crystal display in which the display operation is carried out in reflection mode and transmission mode.
2. Description of the Related Art
Liquid crystal displays (LCDs) have become displays of choice among the various developed flat panel type displays because they are much slimmer and lighter than other types of displays. They also require lower driving voltage and lower power consumption.
LCD displays are classified as transmission type which display images using an external light source such as a backlight, as reflection type which display an image using natural light, and transmissive and reflective type which display in a transmission mode using an internal light source provided in the display itself at indoors or a dark place where an external light source does not exist and the display operates in a reflection mode to display images by reflecting external incident light in a high brightness environment such as at outdoors.
LCDs can also be classified depending on the way they are driven. For example, In the passive matrix type, pixels in the LCDs are driven using a root-mean-square (rms) of a difference between voltages applied to signal lines and scanning lines, while a line addressing in which a signal voltage is applied to all of the pixels at the same time is carried out. In the active matrix type, pixels are driven by a switching element such as a MIM (Metal-insulator-metal) device or a thin film transistor.
FIG. 1 is a sectional view of a conventional transmissive and reflective type LCD, and shows an active matrix type LCD using the thin film transistor.
Referring to FIG. 1, the conventional transmissive and reflective type LCD includes a first substrate 10, a second substrate 40 arranged facing the first substrate 10, a liquid crystal layer 50 formed between the first substrate 10 and the second substrate 40, and a light source, i.e., a backlight assembly 60 disposed at a rear side of the first substrate 10.
The first substrate 10 includes a first insulating substrate 11, a thin film transistor 25 formed on the first insulating substrate 11, a passivation film 30 having a contact hole 32 for exposing a part of the thin film transistor 25, a transparent electrode 34, and a reflection electrode 36. The thin film transistor 25 includes a gate electrode 12, a gate insulating film 14, an active pattern 16, an ohmic contact pattern 18, a source electrode 20, and a drain electrode 22. The transparent electrode 34 functions as a pixel electrode for transmitting light that is generated from the backlight 60 and is then incident through the first substrate 10. The transparent electrode 34 is connected to the thin film transistor 25 formed on every unit pixel region on the first substrate 10. The reflection electrode 36 reflects external light that is incident through the second substrate 40 and at the same time functions as another pixel electrode. The transparent electrodes 34 include regions of a transmission part T and a reflection part R for reflecting the external light incident through the second substrate 40.
The second substrate 40 includes a second insulating substrate 42, a color filter 44 comprised of RGB pixels for displaying colors while light is transmitted therethrough, a black matrix 46 for preventing the light from being leaked between the pixels, and a transparent common electrode 48.
The liquid crystal layer 50 is made of 90° twisted nematic (TN) liquid crystal, and has an approximately 0.24 of Δnd which is a product of anisotropy Δn in refractive index and thickness d of the liquid crystal layer 50.
Also, according to an alignment direction of the liquid crystal molecules, a first polarizing plate 54 and a second polarizing plate 58 are respectively attached to external surfaces of the first and second substrates 10 and 40 so as to transmit only polarized light in a specific direction. The first and second polarizing plates 54 and 58 are all linear polarizers in which each polarizing axis of the first and second polarizing plates 54 and 58 is orthogonal to each other.
Between the first substrate 10 and the first polarizing plate 54, and between the second substrate 40 and the second polarizing plate 58, there are respectively arranged a first ¼ wavelength phase difference plate 52 and a second ¼ wavelength phase difference plate 56. Each of the ¼ wavelength phase difference plates 52 and 56 functions to convert linearly polarized light to circularly polarized light, or vice versa by causing a phase difference of ¼ wavelength between two polarization components that are orthogonal to each other and are parallel to the optical axes of the ¼ wavelength phase difference plates 52 and 56.
Hereinafter, there are respectively described operations in the reflection mode and the transmission mode in the conventional transmissive and reflective type LCD shown in FIG. 1.
FIGS. 2A and 2B are schematic views for illustrating an operation of the conventional LCD in the reflection mode.
First, when a pixel voltage is not applied (OFF), as shown in FIG. 2A, light that is incident from an outside is transmitted through the second polarizing plate 58, so that the light is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate 58. The linearly polarized light is transmitted through the second ¼ wavelength phase difference plate 56, so that the linearly polarized light is converted onto left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the liquid crystal layer 50, so that the left-handed circularly polarized light is linearly polarized in a direction vertical to the polarizing axis of the second polarizing plate 58, and is then incident onto the reflection electrode 36. The linearly polarized light, which is reflected by the reflection electrode 36, is transmitted through the liquid crystal layer 50, so that the linearly polarized light is converted onto the left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the second ¼ wavelength phase difference plate 56, so that the left-handed circularly polarized light is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate 58. And then, the linearly polarized light is transmitted through the second polarizing plate 58, so that a white image is displayed.
When a maximum pixel voltage is applied (ON), as shown in FIG. 2B, light that is incident externally is transmitted through the second polarizing plate 58, so that it is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate 58. The linearly polarized light is transmitted through the second ¼ wavelength phase difference plate 56, so that it is converted onto left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the liquid crystal layer 50 without variation in the polarization state, and is then incident onto the reflection electrode 36. The light, which is incident onto the reflection electrode 36, is reflected by the reflection electrode 36, so that it is converted to right-handed circularly polarized light and the converted right-handed circularly polarized light is transmitted through the liquid crystal layer 50. Thus, the right-handed circularly polarized light, which has been passed through the liquid crystal layer 50, is transmitted through the second ¼ wavelength phase difference plate 56, so that it is linearly polarized in a direction perpendicular to the polarizing axis of the second polarizing plate 58. The linearly polarized light is shielded by the second polarizing plate 58, so that a black image is displayed.
FIGS. 3A and 3B are schematic views for illustrating an operation mechanism of the transmission mode.
When a pixel voltage is not applied (OFF), as shown in FIG. 3A, light that is irradiated from a backlight disposed below the first polarizing plate 54 is incident onto the first polarizing plate 54, and only light propagating in a direction parallel to the polarizing axis of the first polarizing plate 54 is transmitted through the first polarizing plate 54. At this time, since the polarizing axis of the first polarizing plate 54 is perpendicular to that of the second polarizing plate 58, the light that has been passed through the first polarizing plate 54 is converted onto light linearly polarized in a direction perpendicular to the polarizing axis of the second polarizing plate 58. The linearly polarized light is converted onto a right-handed circularly polarized light by a first ¼-wavelength phase difference plate 52. The right-handed circularly polarized light is transmitted through a transparent electrode 34, and is then incident to a liquid crystal layer 50. The right-handed circularly polarized light is transmitted through the liquid crystal layer 50, so that it is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate 58. The linearly polarized light is transmitted through a second ¼-wavelength phase difference plate 56, so that it is converted onto the right-handed circularly polarized light. At this time, since only a light component propagating in a direction parallel to the polarizing axis of the second polarizing plate 58 can be transmitted through the second polarizing plate 58, only about 50% of the right-handed circularly polarized light is transmitted through the second polarizing plate 58. Accordingly, there is a light loss of about 50%, and an image having a moderate brightness is displayed.
Meanwhile, although not shown in the drawings, an optical path of the incident light becomes different at a region where a metal layer, such as the gate line, the data line, or the reflection electrode exists in the transmission mode. In other words, light that is incident from the backlight is transmitted through the first polarizing plate 54, so that it is linearly polarized in a direction parallel to the polarizing axis of the first polarizing plate 54. The linearly polarized light is transmitted through the first ¼ wavelength phase difference plate 52, so that it is right-handed circularly polarized. The right-handed circularly polarized light is reflected by metal layers, and become left-handed circularly polarized. Then, the left-handed circularly polarized light is transmitted through the first ¼ wavelength phase difference plate 52, so that it is linearly polarized in a direction parallel to the polarizing axis of the first polarizing plate 54. Accordingly, the linearly polarized light is absorbed in the first polarizing plate 54, and does not return to the backlight. Thus, the light reflected by the metal layers is not reproduced and disappears, so that an overall light efficiency is lowered.
When a maximum pixel voltage is applied (ON), as shown in FIG. 3B, light that is irradiated from a backlight disposed below the first polarizing plate 54 is incident onto the first polarizing plate 54, so that only light propagating in a direction parallel to the polarizing axis of the first polarizing plate 54 is transmitted through the first polarizing plate 54. The light linearly polarized by the first polarizing plate 54 is converted into a right-handed circularly polarized light after being transmitted through the first ¼ wavelength phase difference plate 52. The right-handed circularly polarized light is transmitted through the transparent electrode 34, and is then incident onto the liquid crystal layer 50. The right-handed circularly polarized light is transmitted through the liquid crystal layer 50 without variation in the polarization state, and is linearly polarized in a direction orthogonal to the polarizing axis of the second polarizing plate 58 after being transmitted through the second ¼ wavelength phase difference plate 56. Afterwards, the light linearly polarized in the direction orthogonal to the polarizing axis of the second polarizing plate 58 is not transmitted to the second polarizing plate 58, so that a dark image is displayed.
As described above, since the conventional transmissive and reflective type LCD has to be provided with the wide band ¼ wavelength phase difference plates 52 and 56 covering an overall frequency band of the visible ray, as well as the first and second polarizing plates 54 and 58 with respect to each of the first and second substrates 10 and 40, manufacturing cost is increased as compared with the transmission type LCD. Also, since the polarization characteristic in the transmission mode causes light loss of about 50%, there are drawbacks in that a light transmissivity decreases by about 50% and contrast ratio (C/R) is lowered.
Further, since Δnd of the liquid crystal layer 50 is only about 0.24 μm which is a half of Δnd (about 0.48 μm) of the conventional transmission type LCD, the cell gap of the liquid crystal cell should be decreased to a level of about 3 μm, and the refractive anisotropy Δn of the liquid crystal also should be decreased. Accordingly, there is a need for a transmissive and reflective type LCD device and method which avoids aforementioned problems.